For a large number of electronic ballasts for fluorescent lamps, current standards require identification and disconnection of the lamps at the end of their life (“end-of-life” disconnection).
A protective circuit for disconnecting the fluorescent lamp at the correct time is known from DE 101 08 138 A1. As long as the fluorescent lamp is still not defective, a criterion is evaluated which results in a disconnection of a half-bridge arrangement at the correct time prior to overheating in the filament region (risk of fusing of the lampholder). In the case of the fluorescent lamp, the filaments are covered by emitters in order to reduce the work function of the electrons. At the end of life of the fluorescent lamp, an emitter which is increasingly absent on an electrode (lamp filament) results in the work function increasing and therefore the voltage drop across an output capacitor being altered.
During normal operation, i.e. before the end of life of the fluorescent lamp is reached, the potential at two reference points in the circuit is compared. Thus, the two reference points on average are at a potential which corresponds to half the voltage which is made available by the DC voltage source.
Towards the end of life, the potential at one of the reference points changes, and this results in a voltage difference between the two reference points. An evaluation circuit detects these changed voltage conditions, wherein a threshold value can be set above which deactivation of the half-bridge arrangement takes place.
In order to prevent an end of life being identified and the fluorescent lamp being disconnected erroneously, the abovementioned asymmetrical state is awaited for a predetermined period of time (for example approximately 20 seconds) and only then is the disconnection of the fluorescent lamp introduced.
Thus, a correctly operating electronic ballast identifies an asymmetry, waits for the predetermined period of time and then switches off the fluorescent lamp.
Since an erroneously produced electronic ballast can result in premature disconnection of the device (complaint) or else faulty operation (no disconnection, safety risk), it is necessary and conventional to also test the electronic ballast in an assembled state critically and uncritically in each case in two directions of asymmetry.
If the 20 seconds mentioned above by way of example for an asymmetry test are used, the tests in both directions (any electrode or filament of the fluorescent lamp can bring about a shift in potential, but in a different direction) on their own result in a test duration of 40 seconds.
Such long test times correspondingly increase the production costs.